1. Field of the Invention
The present invention relates to a combustion system for use with solid combustibles, and more particularly to combustion systems and to their operation control methods for use with waste which have unknown stoichiometric air-to-fuel ratios as fuels.
2. Description of the Related Art
FIG. 12 is a cross-sectional view of a conventional gasifying combustion incinerator (a dry distillation incinerator) disclosed in a publication (e.g., "A Hundred Pieces of Selected Waste Treatment Techniques," in 1993, by the Kankyo Kogai Shinbun Co., Ltd.). In the drawing, reference numeral 1 designates a waste inlet which doubles as a safety valve; 2a designates a dry distillation incinerator (a thermal decomposition incinerator); 2b designates an incinerator installed separately from the distillation incinerator 2a; 4 designates a dry distillation air vent for supplying dry distillation air to the dry distillation incinerator 2a; 6 designates a dry distillation air chamber of the dry distillation incinerator 2a; 9 designates a combustion space; 11a designates a burner for starting the dry distillation incinerator; 11b designates a burner for starting the incinerator 2b; and 12 designates a combustion air vent. Reference numeral 14 designates a combustion air chamber of the incinerator 2b which communicates with the inside of the incinerator 2b through the combustion air; 15 designates a dry distillation gas flow channel (for thermally-decomposed gases) which permits communication between the dry distillation incinerator 2a and the incinerator 2b; 101a and 101b designate temperature sensors; 102a, 102b, and 102c designate airflow control valves.
The operation of the foregoing conventional gasifying combustion incinerator will be described. First, combustible waste is fed into the dry distillation incinerator 2a through the waste inlet 1, and dry distillation air is fed into the dry distillation incinerator 2a through the dry distillation air chamber 6 and the dry distilled air vent 4. A combustion-supporting oil is fed to the starting burner 11a, and partial combustion of the waste is initiated inside the base of the dry distillation incinerator 2a. The adjoining portions of the waste are heated by the heat of the combustion, and partial combustion of the waste progresses continuously in the insufficient quantity of air in an upward direction. At this time, dry distillation combustible gases (hereinafter referred to as "thermally-decomposed gases") which contain a large quantity of unburned gas develop in the dry distillation incinerator 2a, and these gases are fed to the incinerator 2b via the thermally-decomposed gas flow channel 15. Since the dry decomposed gas developed immediately after the initiation of the dry distillation contains a small proportion of combustible components, the combustion of the gas is supported by the starting burner 11b in the incinerator 2b. After full-scale generation of thermally-decomposed gases and sufficient heating of the inside of the combustion chamber 9 have been achieved, the thermally-decomposed gases are mixed with combustion air which is introduced into the combustion chamber 9 via the combustion air chamber 14 and the combustion air vent 12. The thus-mixed gas causes spontaneous combustion, and the starting burner 11b is stopped at this time.
The combustion in the incinerator 2b is controlled so as to make the temperature of the combustion gas stable by the detection of the temperature of the combustion gas developed in the incinerator 2b through use of the temperature sensor 101b, and by the regulation of the rate of flow of distilled air into the dry distillation incinerator 2a and the rate of combustion air flowing into the incinerator 2b by the respective airflow control valves 102b and 102c.
FIG. 13 is a cross-sectional view showing the structure of a conventional stoker fired furnace disclosed in; e.g., Unexamined Japanese Patent Application No. Hei-6-213423. In the drawing, reference numeral 1 designates a hopper which is a waste inlet for an incinerator 2; 3 designates a pusher for feeding the waste fed in the hopper 1 into the incinerator 2; and 4 designates stokers or grates for drying, burning, and post-burning of the waste, in which they are classified as a drying stoker 4a, a burning stoker 4b, and a post-burning stoker 4c in the order from the one being closest to the pusher 3. Reference numeral 5 designates a primary air blower for supplying primary air to the stokers 4a to 4c; 6 designates a primary air flow channel which permits communication between the lower portions of the stokers 4a to 4c and the primary air blower 5; 7 designates a burned ash inlet into which ash resulting from the burning of the waste in the stoker 4c is fed; and 9a, 9b designate combustion spaces above the stokers 4, i.e., freeboard, wherein 9a is a primary combustion area, and 9b is a secondary combustion area. Reference numeral 11 designates a starting burner; 12 designates a secondary blower for supplying secondary air to a secondary combustion area 9b; 13 designates a monitoring camera for observing the state of combustion of the waste on the stokers 4a to 4c; 14 designates a waste heat boiler; 15 designates a turbogenerator; and 16 designates an exhaust gas processing facility.
Next, the operation of the foregoing stoker fired furnace will be described. At the time of starting-up of the stoker fired furnace, waste is introduced into the hopper 1. The accumulated waste is fed from its bottom to the stokers 4 by the pusher 3. The waste supplied onto the stokers 4 is fed in order from the drying stoker 4a to the burning stoker 4b. At this time, the primary air is supplied to the base of the respective stokers 4a, 4b, and 4c from the primary air blower 5 by way of the primary air flow channel 6. The starting burner 11 is then activated so as to ignite the waste held on the stokers 4a to 4c. The waste held on the burning stoker 4b is burned, and then the thus-burned waste is fed to the post-burning stoker 4c by virtue of the movement of the stoker 4b. At the same time, new waste is fed to the drying stoker 4a by the pusher 3.
An unburned-component-contained gas resulting from the partial combustion of the waste in the insufficient quantity of air on the burning stoker 4b is substantially completely burned by introducing secondary air supply into the secondary combustion area 9b from the secondary air blower 12. Thermal energy from the combustion of the gas is converted into thermal energy of steam by the waste heat boiler 14 disposed downstream from the secondary combustion area 9b. The thus-converted thermal energy is further converted into electrical energy by; e.g., the turbogenerator 15. The exhaust gas processing facility 16 removes fly ash and acid gas from the combustion gas that has passed through the waste heat boiler 14. The waste that is in flames is sent to the post-burning stoker 4c from the burning stoker 4b where it is completely reduced to ashes, and the resultant ashes are supplied to the burned ash inlet 7.
The state of combustion in the incinerator 2 is monitored by a combustion gas temperature monitor (not shown), the concentration of oxygen in the exhaust gas, or the positions of flames which develop on the burning stoker 4b and are observed by the monitoring camera 13. The combustion of the waste is controlled by regulating a feed rate of waste to the stokers 4 and the flow rates of the primary and secondary air such that complete combustion of the waste fired on the burning stoker 4b and a predetermined concentration of oxygen in the exhaust gas are achieved, and constant thermal load is imposed on the waste heat boiler 14.
FIG. 14 is a cross-sectional view illustrating the structure of a fluidized bed furnace disclosed in the publication (e.g., "Practical Designing of a Fluidized Bed Furnace," the enlarged and revised edition, on Aug. 20, in 1994, by the Kogyo Shuppan Co. Ltd.). In the drawing, reference numeral 2 designates the main unit of a fluidized bed furnace; 3 designates a waste feeder; 4 designates a fluidized bed; 6 designates a fluidized air inlet; and 61 designates a fluidized air chamber. Reference numeral 62 designates a distribution plate, and sand which serves as a bed material on top of the distribution plate 62. Reference numeral 7 designates an incombustible extraction pipe provided underneath the fluidized bed 4; 8 designates an incombustible extraction device; 81 designates a vibrating screen for separating incombustible from fluid sand; 82 designates a fluid sand circulation system; 9 designates a freeboard formed above the fluidized bed 4; 10 designates an auxiliary fuel supply gun; 11 designates a starting burner; and 12 designates a secondary air nozzle for supplying secondary air to the freeboard 9.
Next, the operation of the fluidized bed furnace will be described. Fluidized air (which doubles as primary air) which is used for constituting a fluid layer is guided from the fluidized air inlet 6 to the inside of the fluidized bed furnace 2 via the fluidized air chamber 61 and the distribution plate 62. The sand accumulated on the distribution plate 62 forms a fluid layer because of the fluidized air, and the fluid layer is heated by the starting burner 11. When the temperature of the fluid layer reaches a temperature (of about 700 degrees centigrade) which is suitable for the combustion of the waste, the waste feeder 3 feeds waste onto the fluidized bed 4, and the waste is immediately dried, thermally decomposed, and partially burned. The resultant combustible gases (hereinafter referred to as thermally-decomposed gases) are mixed with the secondary air introduced through the secondary air nozzle 12 within the freeboard 9 above the fluidized bed 4. The waste is substantially burned completely. Incombustible left in the fluidized bed 4 are extracted by the incombustible extraction device 8 by way of the incombustible extraction pipe 7. The extracted materials are divided into sand and incombustible, and the sand is returned to the fluidized bed by way of the fluid sand circulation system 82.
The waste is vigorously mixed with hot sand of the fluidized bed 4 in the fluidized bed furnace, thereby providing a high reaction rate and leading to drying, thermal decomposition, and partial burning of the waste within a short period of time. For this reason, there is a tendency for the fluidized bed furnace to be apt to incompletely burn waste if there are variations in the quantity and quality of the waste. For example, if there is an increase in a proportion of plastic materials in the waste, a shortage in the combustion results in a hike in the concentration of CO in the exhaust gas.
To prevent such a problem, there is another example contrived to suppress the incomplete combustion of waste by partially fluidizing the fluidized bed 4 so as to make the reaction mild (refer to a publication entitled "A Collection of Research Papers Presented at the 12th National City-cleaning Workshop," February 1992). However, this method also fails to provide sufficient countermeasures against variations in the quality of solid waste.
The following are examples of conventional combustion control methods, and the items to be measured and the control to be used are detailed below.
Unexamined Japanese Patent Application No. Hei-7-133917
Items to be measured: the quantity of combustion air, the concentration of oxygen in an exhaust gas, and the temperature of the exhaust gas, PA1 Items to be controlled: the quantity of combustion air, a feed rate of refuse, a rate of travel of the waste between stokers, and the diffluence of the combustion air, PA1 Items to be measured: the volume and weight of waste within a hopper, PA1 Items to be controlled: an increase or decrease in the supply of waste, combustion, and the processing of flue gas, PA1 Items to be measured: the temperature of air supply, the temperature of a fluid layer, the temperature of the exhaust gas, a flow rate of primary air, and a flow rate of secondary air, PA1 Items to be controlled: a flow rate of and a distribution ratio between the combustion air in a fluid layer and combustion air in the freeboard, PA1 Items to be measured: the brightness of the inside of an incinerator, and the concentration of oxygen in the exhaust gas, PA1 Items to be controlled: a feed rate of garbage, and a feed rate of combustion air, PA1 Items to be measured: a load current of a motor used for driving waste supply means, and the temperature of gas within an incinerator, PA1 Items to be controlled: a flow rate of loading of materials to be burned, and a flow rate of secondary air, PA1 Item to be measured: a burn-off point through use of an infrared ray, PA1 Items to be controlled: a travel speed of waste, and a feed rate of air supply, PA1 Item to be measured: the concentration of specific components in the exhaust gas, the components developing in a post-burning zone of a stoker, PA1 Item to be measured: a feed rate of waste, PA1 Item to be measured: images of the inside of an incinerator (images of the flames), PA1 Item to be measured: images of the inside of an incinerator (the distribution of brightness within the incinerator) (detection of the position of combustion and a burn-off point), PA1 solid combustibles supply means; PA1 a thermal decomposition section which generates combustible gases by thermally decomposing or partially burning the solid combustibles received from the solid combustible supply means; PA1 a combustion section which burns the combustible gases generated by the thermal decomposition section; PA1 first air supply means which supplies air to heating means for heating the thermal decomposition section or to the thermal decomposition section; PA1 second air supply means which supplies air to the combustion section; and PA1 thermally-decomposed gas quality detection means for detecting the quality of the combustible gases generated in the thermal decomposition section. PA1 detecting the quantity of combustible gases developed in the thermal decomposition section by means of the thermally-decomposed gas quantity detection means; PA1 detecting a stoichiometric air-to-fuel ratio or a quasi stoichiometric air-to-fuel ratio of the combustible gases by means of the thermally-decomposed gas quality detection means; and PA1 supplying to the combustion section the quantity of air which is obtained by multiplying the product of the thus-detected quantity of combustible gases and the stoichiometric air-to-fuel ratio or quasi stoichiometric air-to-fuel ratio, by a predetermined factor by means of the second air supply means. PA1 detecting a flow rate of air supplied to the thermal decomposition section by means of the airflow rate detection means; PA1 detecting a stoichiometric air-to-fuel ratio or quasi stoichiometric air-to-fuel ratio of the combustible gases developed in the thermal decomposition section by means of the thermally-decomposed gas quality detection means; PA1 calculating the quantity of the combustible gases by multiplying the thus-detected flow rate of air by a predetermined factor; and PA1 supplying to the combustion section the quantity of air which is obtained by multiplying the product of the quantity of combustible gases and the stoichiometric air-to-fuel ratio or quasi stoichiometric air-to-fuel ratio, by a predetermined factor by means of the second air supply means. PA1 detecting the temperature of the combustible gases developed in the thermal decomposition section by means of the thermal decomposition section temperature detection means; PA1 detecting a stoichiometric air-to-fuel ratio or quasi stoichiometric air-to-fuel ratio of the combustible gases developed in the thermal decomposition section by means of the thermally-decomposed gas quality detection means; and PA1 changing at least either the feed rate of the solid combustibles by means of the solid combustibles supply means or the feed rate of air by the first air supply means, on the basis of variations in the thus-detected temperature of the combustible gases and in the stoichiometric air-to-fuel ratio or quasi stoichiometric air-to-fuel ratio of the combustible gases. PA1 detecting the temperature of the combustible gases developed in the thermal decomposition section by means of the thermal decomposition section temperature detection means; PA1 detecting a stoichiometric air-to-fuel ratio or quasi stoichiometric air-to-fuel ratio of the combustible gases developed in the thermal decomposition section by means of the thermally-decomposed gas quality detection means; and PA1 changing at least either the feed rate of the solid combustibles by means of the solid combustible supply means or a heating rate of heating means, on the basis of variations in the thus-detected temperature of the combustible gases and in the stoichiometric air-to-fuel ratio or quasi stoichiometric air-to-fuel ratio of the combustible gases.
Unexamined Japanese Patent Application No. Hei-7-119946
Unexamined Japanese Patent Application No. Hei-6-341629
Unexamined Japanese Patent Application No. Hei-7-167419
Unexamined Japanese Patent Application No. Hei-6-74435
Unexamined Japanese Patent Application No. Hei-6-331122
Unexamined Japanese Patent Application No. Hei-6-288529
Unexamined Japanese Patent Application No. Hei-7-39854
Unexamined Japanese Patent Application No. Hei-6-86926
Unexamined Japanese Patent Application No. Hei-7-55125
As described above, combustion control based on the measurement of a feed rate of waste, the quantity of combustion air, the temperature of combustion air, the temperature of an exhaust gas, the concentration of oxygen in the exhaust gas, the concentration of specific components in the exhaust gas, and images of the inside of an incinerator.
A conventional solid waste combustion system has the aforementioned structure and is operated in the previously-described manner. With regard to the combustion control, there are some examples in which the quantity of waste is roughly ascertained, similar to the previous examples. However, there are no examples in which variations in the quality of waste are previously detected, and control suitable for those variations is not effected in the current state of the art. Particularly, the quality of waste, more specifically stoichiometric air required to burn the waste (namely, the optimum quantity of air used in burning fuel) is not ascertained at all. As a result of this, a suitable quantity of air is not supplied in response to variations in the quality of supplied waste, thereby resulting in a sharp increase in the concentration of CO in the exhaust gas as well as an increase in the temperature of combustion gas developed in the incinerator. Further, this causes variations in the temperature of steam in a boiler subjected to thermal load, as well as an increase in the concentration of CO in the exhaust gas leading to the discharge of deadly poisonous dioxins.
Some of incinerators have recently begun to adopt fuzzy control in which a conceptual quantity that cannot have been quantified by a conventional technique is converted into numbers by unification and combination of various types of information about quantities related to the incinerator through fuzzy inference, thereby achieving improvements in controllability. However, it takes a long period of time to develop know-how related to operations of the incinerator into fuzzy inference. Recent fuzzy control allows stabilization of combustion by regulating a feed rate of waste which can be burned in the incinerator. In contrast, it cannot cope with drastic variations (or a change for the worse; e.g., an increase in water content) in the state of the art.